In reply to Donald M:
I think the 'rules of the air' should apply. Basically, the faster, and therefore more mobile has the obligation to avoid collision. For example, an aeroplane can manoeuvre out of the way of an airship, however it would be difficult for the opposite.
In this case, it would be the skier who would have the obligation to avoid collision. A climber, for example, may be stuck at a 'crux' (even a grade I gulley could be at someone's limit).
That rule would also imply that a skier would ski within appropriate 'limits' along the lines of the skiing guidelines someone posted above.
In the air, of course, there is a 'standard' way to turn to avoid collision, that wouldn't be possible for a skiier. However, bearing in mind that skiing in a gulley would also klikely generate 'falling hazards, then it would seem that a skier would be obliged to halt and wait for a climber to move to a place of safety before moving on.